The commercial recovery of particularly pure, sublimable, organic materials, i.e. such materials which under the working conditions pass over immediately from the gaseous state into the solid state, from a hot gas mixture, which is mainly taken from a reactor for the production of said materials, and which contains as impurities side products which, under the required working conditions, remain in gaseous form in the exhaust gas, has gained in importance in recent years.
The separation of the sublimable materials from these gas mixtures containing them is known. All known methods have in common that the hot gases must be cooled off to such a degree on cooled surfaces that the desired separation degree for the substance to be obtained is achieved. In this connection, the heat exchange surfaces often get coated with products so that a periodic removal is necessary. Large sublimation chambers can easily be emptied with scrapers or brushes. However, due to the relatively small chamber surfaces, the throughput or area-time-yield is low. The most economical separators at the present time are ribbed pipe condensers which, however, cannot be emptied mechanically. The recovery of the sublimate or condensate takes place here by periodic heating up of the ribbed pipes to temperatures above the hardening point of the products recovered and the withdrawal of the melt into storage bins. The disadvantage in this method consists in the requirement of two condensers which have to be operated alternately, of which one is being loaded while the other is melted off. Furthermore, such a condenser is unsuitable for products which have a very high melting point and/or which disintegrate at their melting point.
Pyromellite acid dianhydride (PMDA), among others, belongs to the products which cannot be obtained in the ribbed pipe condenser of the customary construction. On the one hand, the melting point of 285.degree. C. is so high that the melting off would create technical difficulties, on the other hand, a discoloration of the product appears when the PMDA is heated to its melting temperature, which cannot be neutralized according to hitherto known methods except through distillation, which due to the high melting point of the product creates technical problems which are also not negligible.